Virtually all electronic devices operate on power which must be of substantially constant, if not very closely regulated voltage and thus require a power supply capable of delivering power at such voltage(s), often at relatively high current. However, in many such devices, current can fluctuate very rapidly. The voltages required can also differ widely between such devices and within a given device. For example, modern microprocessor chips, when operating at full clock speeds may require hundreds of amperes at a fraction of a volt on a given chip and yet require voltages of several volts to communicate between chips while presenting a very light load and draw low current levels when in an idle or stand-by state while being able to shift between idle of stand-by states and full operation very quickly and often. Similarly, memory chips generally require little power during normal operation but may require substantial power for periodic refresh which must be performed rapidly and often.
The increasing popularity of portable devices such as laptop computers, smart phones, personal digital assistants and panel type computers, particularly of the hand-held type, has put increasing demands on the design and manufacture of power supplies, generally referred to as power converters, voltage regulators or the like. For such devices, small size, light weight and a high level of functionality are extremely desirable. Therefore, it is considered imperative to develop and use power converters for such devices that are as small and light weight as possible since the power converter(s) do not directly contribute to the ultimate functionality of the device. Conversely, the need for limitation of size and increase of power density of power converters is, if anything, greater than the need for size limitation and increased functionality in such devices since any volume reduction in the required power converter(s) can be used for providing increased functionality of the portable device.
Additionally, the requirement for close voltage regulation at very low voltages and very high currents has led to the concept of point-of-load power converters so that the power connections may be kept very short to limit voltage drop in the power connections and the impact of parasitics and to limit the propagation time of signals from the load to control operation of the power converter. Since the semiconductor chips and other loads may be very small, there is increased pressure to produce power converters of comparable sizes in order to place them as closely as possible to the device being powered.
Additionally, among the many power converter topologies that are known, resonant converters and/or so-called buck/boost converters, in particular, have drawn substantial interest for portable devices. While these topologies are not of minimal complexity, they are still favored for efficiency and the ability to extract more power from rechargeable battery power sources since they can regulate their outputs at voltages above the voltage of the input power source which diminishes as battery charge is drawn down.
While much effort has been expended in recent years to provide power converters of reduced size and increased power density, none has satisfied all power converter requirements for hand-held devices of current and foreseeable designs. For example, chip level integration placing a magnetic core and winding on a silicon substrate is applicable only to power converters limited in output current to below one ampere which is adequate for some hand-held devices but not for others such as netbooks and tablets. Thus, system level integration is preferred for high current POL modules. Very high frequency switching devices based on gallium nitride (GaN) combined with low temperature co-fired ceramic (LTCC) ferrite inductor substrates have reached power densities of 1 Kw/in3 but are too expensive, the technology of GaN devices is not sufficiently mature and the high temperatures (˜900° C.) required for sintering ferrite materials complicates manufacturing and raises costs too greatly for high volume manufacture.